The Easter Egg – everyone can (visually) dive into Earth's past

As my first Easter Egg post, I advertised the most important French hebdomaire, weekly newspaper, Le Canard enchainé (lit. the Chained Duck), something you can't find in the virtual world. This year it is something more peculiar, literally a virtual (version of our) world. Well, worlds, the bygone ones.

Why non-geologists need to know how the world looked like 

There are many papers dealing with 'phylogeography', the spatial history of animals or plants, how their lineages unfolded in space and time. And quite a few programmes and methods to infer or test what we call 'explicit biogeographic scenarios'. In biogeographers' textbooks of the 90s and Zeroes, you could read that to infer a biogeographic history of e.g. a plant genus, you don't need any fossils or historical information. Thus, it's rather unsurprising that the results of most inferences were either trivial or pointless — the reconstructed 'ancestral area', the putative place of origin, was entirely ambiguous. However, instead of "we have no idea at all", you'd read the oddest and funniest things (there are a few recently published examples in my bad science category).

These days, it has become fashion to control the inference using a dated phylogenetic tree. In fact, the selling point of e.g. the DEC method (Ree & Smith 2008) was that you can give probabilities for dispersals at a certain time – the chance a lineage can expand into a new area (in contrast to the simple splitting of a lineage into two related to the area it covers, which is called "vicariance").

The two basic phylogeographic patterns. Trivial when you have a good fossil (or historical) record, near-impossible to distinguish when you have only modern-day data; why intelligent people came up with a plethora of more or less sophisticated models and endlessly discussed which one is the best.

This should allow to incorporate prior fossil and tectonic knowledge. The first paper to introduce this new approach won a price but never appeared in print (Moore et al. 2006, submitted to Evolution); it had to be retracted because of fundamental flaws.
[I got into contact with it, as the first author then asked us for our data on the beech they wanted to use as one of the test organisms; our data never made it in the final manuscript, though, they opted to use instead outdated but easier data, because, as I told warned him when sending the data, even the fanciest of methods would collapse on the backdrop of the complex signal), and it did.] 
Ree & Smith (two of the four co-authors of Moore et al.) demonstrated later the superiority of the new method by assuming that you can't disperse to Hawaii before 5 Ma (setting the probability to near-zero), and, hence, the new method was the only one inferring a non-Hawaiian origin for an exclusively Hawaiian plant lineage. One of many but nice example for the not uncommon ignorance of tectonics and palaeogeography in phylogeographic papers (and lack of common sense), even those explicitly referring to it. Although
  1. the modern islands are very young, the reason for telling the programme that pre-5 Ma Hawaii is a no-go (the divergence was inferred to be older), and 
  2. (by far) the most plants only grow "subaerial" (lit. below the air), i.e. above the high-tide water line, 
Hawaii is only the current point in a long history of islands. The modern islands are only the most recent part of an volcanic hotspot-fuelled 'conveyor belt' of islands: as those in the (now) ENE are eroded and sink, new ones emerge from the ocean floor in the (now) SSW.

Zooming in on the Hawaiian conveyor belt, a chain of sunken islands.

The older a plant lineage, the earlier it could have dispersed into (proto-)Hawaii. Current Hawaii may be only 5 Ma old, but the archipelago has always been sitting in the Pacific waiting to be populated. Once you reached it (150 Ma ago, just stepping over from Kamchatka, 100, 50 or 5 Ma crossing the Pacific), you just have to move from the sinking to the new emerging islands. Indefinitely, it's a very powerful and stable volcanic hotspot (ultimately arriving in South America, maybe). Luckily, your stony conveyor belt approached the equator, so it's even getting warmer (never bad for most plants). Putting a constraint like "can't be on Hawaii before 5 Ma" is simply nonsense (and the conclusion of Ree & Smith remains one hypothesis of many)

Relying on nonsensical palaeogeographic constraints is, however, quite common in studies with explicit phylogeographic inferences. "Long-distance dispersal" was long a selling point and went hand-in-hand with much too young molecular dating estimates (The most common errors regarding node dating) and a general lack of knowledge in neontological circles about Earth's palaeogeography (very few molecular dating or biogeographic papers are reviewed by palaeontologists). Following the lead of zoologists, botanists became soon obsessed with oceans and waterways. But water alone is not the greatest of barriers for plant migration. The most extreme cases are probably plants that live in water, whose seeds easily can get in contact with the feet and feathers of migrating birds. They can have near-identical genotypes in eastern Africa and India as in the case of Aponogeton (see our papers: Grímsson et al. 2014, Chen et al. 2015), in sheer ignorance of the tectonic history and modern-day obstacles such as the, very hostile to most plants, Arabian Peninsula.

But in contrast to animals, plants don't roam. With plants, it's not so much about how to get from A to B via a landbridge, a physical 'subaerial' connection, subsequent island hopping (like in the case of Hawaii), in the feathers or bellies of birds (Aponogeton), blown with the wind (birches and alders, and the enigmatic southern hemispheric Nothofagus), floating by yourself (Cycas, several palms) or on natural rafts. All long-known and -pondered, recently a bit forgotten; see e.g. this nearly 70 years old scientific essay about trans-oceanic plant dispersal with respect to the flora of New Zealand; and Ridley (1930) about the modes of dispersal (including not a few that directly invalidate any model used for 'explicit' biogeographic inferences), a book we had to use several times lacking any newer research along the same lines. It's about whether B is a place to root (quite literally). And, it helps to know where you were, when the plant thrived (10, 20, 50, 100 million years ago).

The proper way to do it – the wonderful world of easy-to-access palaeglobes

When you have a dated tree, and some idea about the fossil record, you just put that into the proper context and you can end up with a biogeographic scenario, better than any (no matter how fancy) programme could possibly infer. The proper context is the past Earth. As a geologist-biologist, when you search for palaeoglobes, globes that show the past Earth, you end up with two principal choices: the works by Ron Blakey (his new page: Deep Time Maps) or Robert Scotese (PALEOMAP project); both equally fantastic and a priceless resource.

Plotting the actual (not inferred) geographic history of the beech (from Denk & Grimm 2009); see also The challenging and puzzling beech – a (hi)story.

For our beech biogeographic history, we went for Blakey's maps because at the time Scotese's were not so easy to access (they differ in aspects, getting a palaeoglobe is as much mathematics as it is an artform), and Blakey provided a special service when you asked him: he would tilt the globe in a way that suits you.

Because his wife insisted (she wrote our invoices, at least, when we used his maps for later papers), he has to make some money with it. You have to pay a modest fee these days to use Blakey's maps in a scientific (i.e. non-profit) publication. I believe every phylogeographic paper should include a figure like the one(s) above, but I suppose when you pay 3000 US$ for gold open access, you can't afford paying 100–200 bucks for a nice digital palaeoglobe providing a real background for your hypotheses and scenarios.

Quite recently, and rather by accident than purpose (while browsing for interest, the benefit of being out-of-science-business), I found Scotese's related project page on ResearchGate Earth History: The Evolution of the Earth System – his entire work is now free and easy to access! A treasure chest for phylogeography. Or anyone interested how our world used to look like.

He also has some nice, easy videos on YouTube about the Earth how it was and how it will become, interesting also for the general public (tested on family members). 

An interview: A Paleogeographer's Song.

The golden egg

And even more recently, I noticed that for some of his time sheets, he now also provides a GoogleEarth layer (kmz) file (here's GoogleEarth's video how to import KML and KMZ files). Now everyone (with a not too crappy computer), specialist scientist or just curious person, can dive into Earth's past.

From whatever angle you want. A few, sparsely annotated, example screenshots (just the bits for the last 80 Ma; all globes within a series are centred on the same point and same zoom; all palaeoglobes: © 2013 Robert Scotese, PALEOMAP project).

Earth's blank side, the Eternal Great Wide Blue
(the always vast Pacific Ocean)

The example of Hawaii shows, there may have always been bits of scattered land in the Great Wide Open that makes our planet a deeply blue one.
Long-known, still often misinterpreted/-presented – 
Australia sailing north (and into the tropics)

Not really an enigma, where (older) Australian tropical plants originally come from – via Malesia, SE Asia, of course. Even during the early Eocene greenhouse phase, Australia, and at its northern tip, New Guinea, was still too south for proper tropics. And a huge ocean separated it from (tropical) south-eastern Asia (Malesia), which started off exactly on the equator 80 million years ago and moved little since then.

Watch the mountains – the Himalayan range as we know it today may be not that old but it's not the first mountain barrier in the region

The land changes, too. Or just shifts position. Mid-latitudes (pics centred on 30 °N latitude, and 90 °E longitude) of Eurasia since the early Eocene (~ 50 Ma).

A rare tilt: Greenland and the Arctic 65 million years ago

Why (some) palaeontologists have to (literally) freeze their asses off – the position of two famous plant fossil localities, the Agatdalen flora is over 60 million years old, and rich in subtropical to temperate forest elements (see e.g. Grímsson et al. 2016a,b; both open access); Ellesmere Island is the place where a bit over ten million years later the first North American oak can be found, thriving within a lush deciduous forest (and not too far from a place where, according to a much-authored Science paper, a dropstone proving an Arctic glaciation nearly was dropped on a crocodile's head – in the supplement of the Science paper it said, the closeness to the crocodile "is not yet fully understood", I guess it must have been a "Woolly Crocodile", and the co-eval lush forest even farther north must be a misinterpretation due to wrongly identified plant fossils).

Biogeographers (neontological) can map their dated biogeographic scenarios on the fitting palaeoglobes. If "absurd" palaeontologists like us can do it, e.g. Grímsson et al. (2017), for Loranthaceae using Blakey's maps, they should be able to do it, too. However, check out the experts adorned re-do for a 1:1 comparison — Liu et al. (2018). And stop just citing outdated work when, e.g. associating every (methodologically too young, lacking proper ingroup fossils) Miocene divergence age for an Eurasian plant lineage with the Himalayan uplift; we don't know for sure how high it was, but the situation was not that different before the Miocene (see pics above).

Peers and editors, who may have less tectonic knowledge than me (and I'm just a graduated geologist-palaeontologist, who worked exclusively as a geneticist and phylogeneticist) such as breast cancer researchers and microbiologists, can easily check how much sense the "explicit inference" really makes.

And palaeontologists can easily trace back where their fossil site really was on a spinning (and evolving revolvable) world. Note, that there is no exact mathematical solution to trace back a particular place in time, you just follow the surrounding basic patterns.

Have fun tilting your palaeoglobe, and all thanks should go to Robert Scotese, for these truly splendid Easter eggs.

Mentioned papers

1 comment:

  1. This is a masterpiece. thanks for sharing the knowledge.


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